Backpack



FIG. 1 is a front and left perspective view of a backpack showing my new design;

FIG. 2 is a rear and left perspective view thereof;

FIG. 3 is a front perspective view thereof;

FIG. 4 is a rear perspective view thereof;

FIG. 5 is a right side perspective view thereof; and,

FIG. 6 is left side perspective view thereof.

The broken lines and faded portions within illustrate portions of the backpack which form no part of the claimed design. 

CLAIM The ornamental design for a backpack, as shown and described. 